In most theaters the scenery is hung from manually operated fly sets. Each set usually consists of a pipe batten hung parallel to the proscenium opening from cables approximately 8-12 ft. apart.
Each cable passes from the pipe batten over loft blocks, mounted on the gridiron or to the underside of the roof beams. From the loft blocks the cables lead over a common head-block, mounted at the side of the stage, usually approximately level with the loft blocks, and down to a counterweight arbor. The counterweight arbor is typically a steel frame, supporting lead, and steel or cast iron weights. It is guided by tracks or cables, which are mounted on the stage sidewall. The weight of the counterweight arbor is balanced to be approximately equal to the weight of the pipe batten and the set hung from the pipe batten.
The fly set is lifted or lowered by an operating rope, tied on top of the counterweight arbor, passing up over and around the head block, down around a tension sheave and up again through a friction lock to the bottom of the counterweight arbor. The friction lock holds the nearly balanced pipe batten and the counterweight arbor in position.
The pipe battens are usually spaced at 6 to 8 inches on centers. The number of sets varies. 20-30 may be used in smaller theaters while more than 100 in larger theaters. The system has several disadvantages. Loading and unloading counterweights for balancing the loads is time-consuming and dangerous. There have been many accidents when counterweights were dropped from a 60-80 feet above the stage. Also, in case of excessive unbalance the fly set may run away when the rope lock is opened.
Motorized winches have been used in larger theaters. In the past most of them have been one of a kind designs, developed specifically for the particular applications. More recently standardized winches have become available, but are still considerably more costly than the manually operated fly sets.
Motorized fly sets are generally spaced wider apart than the manual sets. 8 inch center-to-center spacing has become an accepted standard in the US. The standardized winches presently available are wider than 8 inches and therefore cannot be mounted side by side in a single row. Double row or staggered winch mounting is required for these winches for spacing the fly sets at 8-inch centers.
Some of the standardized fly set winches are zero fleet angle type, meaning that the angle of the cables between the cable drum and the loft block sheaves, mounted on the winch base, does not change when the cables wind or unwind on the drum. This feature is achieved by translating the drum in its longitudinal direction with respect to the sheaves, in synchronization with the back and forth travels of the cables in the drum cable grooves. As an alternate, the drum can be stationary and the loft block sheaves can be translated in similar manner with respect to the drum. This translation can be accomplished by a screw, with the thread pitch identical or in fixed ratio with the spacing of the cable grooves in the drum. The screw can be non-rotatably mounted to the winch base. It would engage a rotating nut, part of the winch. As an option the screw can be rotatably mounted on the winch engaging a fixed nut mounted on the winch base. As another option, the grooves in the winch drum can be used for translating the drum in its longitudinal direction through a device such as a rotating cam or wheel mounted on the winch base and engaging the drum grooves. In this case the drum grooves act as screw thread.
All the existing above described winches are mounted on external base structures and use separate protective enclosures around the winch moving parts.